This invention relates to tuning mechanism for structures that support propagating electromagnetic waves and more particularly to a locking mechanism for fixing the axial position of a threaded tuning or matching screw in a wall of a waveguide.
Various microwave devices including waveguide filters employ a metallic tuning screw in the wall of a waveguide. In the case of a filter, they are used to adjust the operating frequency of the filter. Such devices generally include a threaded bushing that is soldered into a broad wall, for example of the waveguide. The surface of the bushing in the waveguide is normally flush with the inner wall of the latter. A metallic tuning screw is threaded into the opening in the bushing so as to protrude into the waveguide. A conventional locknut is threaded onto the portion of the tuning screw on the outside of the waveguide with opposite faces of the nut being parallel to an adjacent surface on the bushing. At microwave frequencies, the portion of the screw in the waveguide operates as a capacitance. This capacitance is varied by rotating the screw to change the axial length thereof in the waveguide. When the screw is in the desired axial position in the waveguide, the locknut is tightened against the bushing to hold the screw in place. This tuning screw and locknut structure has the disadvantage that the screw may move sideways during tightening of the locknut and thereby detune the filter. If the locknut is partially tightened in order to minimize lateral movement of the screw, further rotation of the latter is rendered difficult. Also, since the mating surfaces of the locknut and the bushing are normal to the axis of the screw, considerable torque is required to ensure that the screw is locked in place. This may cause residual stresses in the threaded junctions that are relaxed with time and temperature cycling so as to produce slight movement of the screw which, in turn, detunes the filter. Although spring-loaded tuning screws provide axial tension during rotation of the screw, the spring also provides the locking action. Such a structure has no firm lateral support and is particularly sensitive to movement of the tuning screw from shock and vibration since it does not employ a locking nut. Tuning screws having precision threads are also available, but they are expensive.